The present invention relates to a tungsten doped crucible and a process for preparing a tungsten doped crucible. More particularly, the present invention relates to a process for doping the inside surface, the outside surface, or both the inside and outside surfaces of a quartz crucible with tungsten to produce a layer which behaves similarly to a bubble free layer upon use in a crystal growing process.
In the production of single silicon crystals grown by the Czochralski method, polycrystalline silicon is first melted within a quartz crucible. After the polycrystalline silicon has melted and the temperature equilibrated, a seed crystal is dipped into the melt and subsequently extracted to form a single crystal silicon ingot while the quartz crucible is rotated. Due to the extremely high temperatures achieved during ingot growth, the quartz crucible is slowly dissolved at the crucible-melt interface as the ingot is grown.
Because of the extreme conditions encountered during ingot growth, high quality quartz glass is an indispensable crucible material in semiconductor technology because of its purity, temperature stability and chemical resistance. Currently, most crucibles utilized in the Czochralski process are produced using a mined quartz sand source. Several different grades of mined quartz are available, with differing alkali purity levels and particle sizes. Regardless of grade, however, these raw materials contain gas formers or precursors in two forms. First, gas can be adsorbed on the surface of the quartz sand. This type of former is generally referred to as a "bubble former" as the adsorbed gas may leave the grain surface at elevated temperatures and coalesce to form large sized bubbles. The second type of gas formers are gases which are soluble within the sand grain itself. Again, at elevated temperatures the solubilized gas can coalesce and form bubbles. Examples of adsorbed and soluble gases in quartz sand include nitrogen, argon, oxygen or other organic type gases. These gas formers, along with bubbles that naturally exist in a fused quartz crucible, can lead to numerous problems in the crystal growing process and result in ingot defects.
Upon use of a quartz crucible in a crystal growing process, gas formers located in the silica matrix at and below the inside crucible surface can coalesce and lead to nucleation and/or growth of bubbles on the crucible surface leading to a reduction in the usable thermal life of the crucible. This bubble nucleation and/or growth on the inside surface of the crucible can lead to formation of pits on the quartz surface, which in turn can lead to the generation of particulates which can also enter the melt and result in the loss of perfect structure of the growing ingot. Crucibles typically used in the ingot growing process may contain about 20 pits/cm.sup.2 on the surface, with each pit measuring approximately 100 to about 400 micrometers in diameter. Also, the bubbles formed can themselves become entrained at the silicon melt-crystal interface, become part of the growing crystal, and result in a void defect in the crystal.
To increase the functionality of quartz crucibles used in a crystal growing process, crucibles used in the Czochralski growth of single crystal ingots typically have two distinct layers. The outer layer in contact with graphite susceptors supporting the crucible contains a high density of bubbles to regulate radiant energy transfer to the melt and growing ingot. The inner layer of the crucible in contact with the silicon melt contains a reduced bubble layer commonly referred to in the art as a "bubble free layer" or "clear layer." This bubble free layer is not completely bubble free, and upon exposure to temperatures typical in the crystal growth process, can degrade through the nucleation and/or growth of bubbles, resulting in a limited lifetime of the crucible and possible degradation of the quality of the growing ingot.
Recently, numerous attempts have been made in the art to produce a bubble free layer substantially free from bubbles on the inside surface of the crucible. Although some improvements have been made in the thermal stability of the inner layer relative to the production of bubbles, bubbles continue to nucleate and/or grow during ingot growth as the processes employed to date have failed to remove all the gases adsorbed on, or soluble within, the quartz grains comprising the crucible. Therefore, a need remains in the art for a quartz crucible having a substantially bubble free layer on the inner surface such that bubble nucleation and/or growth is reduced or eliminated such that ingot degradation does not occur. Also, a need exists in the industry for a quartz crucible having a substantially bubble free layer on the outer surface such that improved chemical stability of the crucible is realized during contact with graphite susceptors during ingot growth.